The present invention relates to a facsimile signal transmission apparatus and, more particularly, to an FCC (Facsimile Control Channel) message transmission apparatus for controlling a protocol of a facsimile signal transmitted from an apparatus which demodulates/modulates and transmits a facsimile signal.
A DCME (Digital Circuit Multiplication Equipment) serving as a highly efficient terminal apparatus increases a channel accommodation capacity for a speech/speech-range data signal about five times using a DSI (Digital Speech Interpolation) technique, a VBR ADPCM (Variable Bit Rate Adaptive Differential Pulse Code Modulation) technique, and a facsimile compression technique. In this DCME, the voice component of a speech signal is defined to be assigned to a transmission channel as a 32/24/16-kbit/s ADPCM signal, a speech band data signal of a modem is defined to be assigned to the transmission channel as a 40-kbit/s ADPCM signal, and a speech band data signal of a facsimile is defined to be assigned to the transmission channel as a facsimile bank constituted by FDCs (Facsimile Data Channels) respectively having transmission rates (CCITT Rec. G. 763, G. 766).
A facsimile compression function is realized on the basis of the arrangement of a facsimile block. That is, an FCC (Facsimile Control Channel) message for realizing facsimile communication control procedures (CCITT Rec. T. 30) is arranged at the start of the facsimile block, and FDCs each having a transmission rate and obtained by demodulating an input facsimile signal are arranged after the FCC message. When the facsimile signal is to be output to the transmission channel, the facsimile signal is assigned as a facsimile bank (4 bits) serving as the transmission unit of the demodulated signal of the facsimile signal.
An FCC message transmission scheme of a conventional DCME is described in detail in the following document: INTELSAT EARTH STATION STANDARDS (IESS) Document IESS-501 (Rev. 3) "APPENDIX B SPECIFICATION FOR THE FACSIMILE DEMODULATION/REMODULATION FUNCTION OF THE DCME" (Recommendation G. 766) 1992, pp. 36-44. The outline of the FCC message transmission scheme of the DCME will be briefly described below with reference to this document.
FIG. 2 shows an arrangement of a DCME with a conventional facsimile signal transmission scheme.
Referring to FIG. 2, on the coding side of the DCME applied with the facsimile signal transmission scheme, a facsimile input signal input from an input terminal 30 is supplied to a demodulator 31 and an identifier 32. The demodulator 31 demodulates the high- and low-speed signals of the facsimile input signal to supply them to a multiplexer 35.
Facsimile transmission control procedures are generally standardized according to CCITT Rec. T. 30. Of these procedures, a binary code signal scheme capable of performing complex operation procedures is mainly applied to a Group 3 digital facsimile. All control signals in the binary code signal scheme are transmitted/received to be arranged in an HDLC (High Level Data Link Control) frame, and a preamble is always transmitted ahead of a binary control signal at the start of the signals. In this case, the preamble represents that the flags ("01111110") of an HDLC frame of 1 second .+-.15% are continuously transmitted when a 300-bit/s signal (low-speed signal) is used.
A training signal (high-speed signal) adjusts a high-speed modem for transmitting a facsimile message. The training signal is constituted by a training sequence defined by the high-speed modem to be used (for example, V. 29 9,600 bits/s).
Unlike the control signal, the facsimile message signal (high-speed signal) is not arranged in HDLC frame, but transmitted as a continuous bit sequence of one page.
As described above, a facsimile input signal must be identified as a low-speed signal (control signal) or a high-speed signal (training signal and facsimile message signal) to perform the facsimile transmission control as described above. For this reason, the identifier 32 identifies the facsimile input signal (low-speed signal or high-speed signal) to supply an identification result to a control signal generator 33. The control signal generator 33 generates FCC messages in accordance with the identification result from the identifier 32 and supplies them to a queue transmitter 34 in a generation order. The FCC messages respectively have priority levels for transmission. The queue transmitter 34 transmits the FCC messages in units of priority levels to supply them to the multiplexer 35. The multiplexer 35 multiplexes a demodulated signal output from the demodulator 31 with an FCC message output from the queue transmitter 34 to transmit the multiplexed signal from, a transmission terminal 36.
On the decoding side of the DCME, a separator 38 separates the multiplexed code signal received from a reception terminal 37, and the separated signals are supplied to a control signal receiver 39 and a remodulator 40, respectively. The control signal receiver 39 controls the remodulator 40 in accordance with the FCC message separated by the separator 38, and the remodulator 40 remodulates the demodulated signal to output it from an output terminal 41 as a facsimile signal.
The facsimile transmission control procedures define that a time of 75.+-.20 ms is required for a switching operation between a low-speed signal (300 bits/s) for controlling a protocol and a high-speed signal (e.g., 9,600 bits/s) for transmitting a facsimile message.
According to an FCC message transmission scheme in the conventional facsimile signal transmission scheme, when FCC messages generated in facsimile communication are transmitted in a priority order, the lower limit of the defined value of 75.+-.20 ms required for a switching operation between a low-speed signal and a high-speed signal cannot be kept in a situation wherein the number of facsimile channels largely changes. In an extreme case, when a given FCC message has a priority level higher than that of an FCC message generated before the given FCC message is generated, these FCC messages are disadvantageously transmitted in a reverse order.